I have a wonderful picture of Eleanor Roosevelt happily knitting a large, heavy sweater on four very long dpns -- they must be at least 14" long.
Whether you can knit a child's sweater on your dpns depends on the length of the needles, the thickness of the yarn, the size of the sweater and the number of stitches you have to knit off squeeze onto each one.
Assuming you have a set of 5 dpns, cast 1/4 of the stitches onto one of them, plus a couple to account for any increases. If the stitches are reasonably spaced, go for it. If they are squeezed together and popping off the needles, you need either longer dpns or circs.
The question of ladders comes up frequently here. Search the topic in the archived threads. One general suggestion: Don't tug on the first stitch you knit off a new needle in an attempt to tighten it. This just exaggerates any tendency to ladder. Second general suggestion: You won't get an obvious line of loose stitches if you knit one or two stitches from each new needle onto the old needle so you're constantly changing the place where the looseness might occur. Third: use a yarn that can be blocked to minimize any unevenness.